Predator, Prey, and Jabberwock

I’m reading through my backlog of Science magazines, and I found this interesting paper titled “7000 years of Emiliana huxleyi viruses in the Black Sea.”It’s really very neat: basically, if you look into the sediments at the bottom of the Black Sea, you can look back in time—the gunk on the top is stuff that floated down recently, while if you dig down a little bit you find gunk that was deposited thousands of years ago.Some of that gunk is minerals, washed into the Black Sea by its tributaries.However, a lot of that gunk is the shells of a type of plankton, Emiliana huxleyi.

What’s neat about this paper, by Marco Coolen from Woods Hole Oceanographic Institution, is that he isolated the DNA of E. huxleyi from that ancient ooze.So, he could look at how the genes in the population changed over the last seven thousand years.Even more interesting, he isolated DNA from the viruses specific to E. huxleyi, so he could look at how their population changed over time.This is nifty, because viruses are really predators.So by looking at the genetic changes in predator and prey over time, it’s possible to see how each changes in response to the other—and also how both respond to changes in the environment, as the Black Sea swung from brackish to fresh and back.

Viruses generally evolve quickly.What Coolen found was that, over 7000 years, the viral DNA he found in the Black Sea muck showed signs of continuous evolution and innovation.A certain combination of viral genes would be dominant for a couple of hundred years, then be replaced by a new combination.Every few hundred years, there would be a completely new dominant virus.

However, the prey—E. huxleyi—evolved more slowly, and revisited certain genetic combinations.While the population might be almost entirely of one sort for a while, it would evolve to be almost entirely of a different sort in a thousand years.It’s as if a population of humans were 99% brown-eyed and 1% blue-eyed, then over a thousand years the population evolved to be 99% blue-eyed.It’s likely that at least some of this change in E. huxleyi was driven by the viruses, as if (in our analogy) there were a virus effective at killing brown-eyed humans but not the blue-eyed.What’s neat is that as the viruses got good at killing the new majority type of prey, rather than evolving into something completely different, the E. huxleyi just reverted back to the original majority.Extending the analogy, it’s as if the viruses became good at killing blue-eyed humans—but rather than the population innovating and evolving to become 99% green-eyed, the population drew upon a small remnant of brown-eyed survivors.These became the parents of a new population that eventually evolved to be 99% brown-eyed.

This is all very neat: it tells us something about predator-prey relationships, the mechanisms of evolution in microbes, and how a very neat and ecologically important organism evolves in response to environmental changes.The paper is also really, really hard to read, freighted as it is with ten-dollar words.These are convenient—it is easier to simply say “coccolith” than “cute little calcite shield made by E. huxleyi,” but it makes for hard reading.After a while, it gets a bit much (and these are just some of the choice words and phrases in a two-page paper):